This invention relates to a torque distribution type differential gear system capable of distributing input torque to a pair of drive shafts at a different magnitude.
Conventional differential gear systems of this type include a housing which is rotationally driven, and a gear train disposed within the housing. This gear train includes a pair of sun gears rotatably received within the housing with its axis in alignment with a rotational axis of the housing, and at least one pair of intermeshing planetary gears which are also engaged respectively with the sun gears. When a torque is supplied to the housing, the torque is transmitted to the respective sun gears through the planetary gears. The torque transmitted to the pair of sun gears are, in turn, transmitted respectively to a pair of drive shafts such as, for example, axles of vehicles.
As the sun gears and the planetary gears to be meshed with the sun gears, there are employed gears having helical teeth such as worm gears or helical gears. Therefore, in the event that a torque is transmitted between the planetary gears and the sun gears, an axial force is produced in the sun gears, thereby the sun gears are urged against the housing or brought closer to each other. As a consequence, when the sun gears are rotated relative to the housing (at the time of differential rotation), frictional resistance is produced between the sun gears and the housing. The input torque is distributed to the pair of sun gears and then to the pair of drive shafts at a different magnitude, depending on degree of this frictional resistance.
It is preferred that the ratio of magnitude of the torque (hereinafter referred to as the "torque bias ratio") to be distributed to the respective sun gears can be properly changed, depending on vehicles in which the differential gear system is used. Thus, in the conventional planetary gear system, as disclosed in German Patent DE 3927071, a single or a plurality of washers are interposed, for example, between sun gears and end walls of a housing, or between a pair of sun gears, so that a desired torque bias ratio is obtained by properly changing various kinds of washers having different frictional coefficients, depending on vehicles.
In the above-mentioned differential gear system, a requirement for a larger torque bias ratio could not be met sufficiently. In order to realize a larger torque bias ratio, there can be contemplated that washers having a large frictional coefficient are used, or washers having an enlarged diameter are used, so that the frictional resistance can be increased.
However, if the former method is employed, frictional resistance per unit area is overly increased between the washers and the sun gears or the housing which is directly in contact with the washers, with the result that hot-seize is likely to occur therebetween. On the other hand, if the latter method is employed, since the diameter of the housing must be increased as the diameter of each washer is increased, the system inevitably becomes large in size.
One approach for solving the above problem is disclosed in Japanese Laid-Open Application No. Hei 1-303339 and Japanese Utility Model Laid-Open Application No. Sho 61-123250. Specifically, a plurality of first washers are unrotatably and axially movably provided on an outer periphery of each drive shaft, and a plurality of second washers are likewise unrotatably and axially movably provided on end walls of a housing. The first and second washers are alternately arranged. As a consequence, frictional interfaces are formed respectively between the end wall of the housing and the first washers, between end faces of the sun gears and the second washers, and between the first washers and the second washers. In this way, the torque bias ratio can be increased by increasing the total area of the frictional interfaces and without increasing the system in size.
However, since the first washers are in engagement with the drive shafts in the differential gear system of the above Laid-Open applications, the differential gear system must be assembled together with the drive shafts and therefore, the assembling is troublesome.
As another example of the prior art, there is a differential gear system disclosed in Japanese Patent Publication No. Sho 63-38586, in which each sun gear has a reduced-diameter sleeve-like portion at its one end. A plurality of first washers are disposed on an outer periphery of the sleeve-like portion such that the first washers are capable of moving axially but incapable of rotation. Similarly, a plurality of second washers are disposed on an inner periphery of a receiving recess formed in an end wall of a housing such that the second washers are capable of moving axially but incapable of rotation. The first and second washers are alternately arranged. Although this system has many frictional interfaces, both sun gears and planetary gears are formed of bevel gears, respectively and a spring is required to be interposed between the sun gears because no axial force can be produced between the sun gears depending on the mode of engagement between the sun gears and the planetary gears. Therefore, this system is different in its basic construction from the differential gear system according to the present invention.